Typically ink-jet printers, or any printers using wet ink, include a pen, also called a printhead, a print zone positioned adjacent the printhead, a feed mechanism for feeding print medium through the print zone, and a platen positioned adjacent the print zone, the platen guiding and supporting the print medium in the print zone during printing.
During printing, ink is placed on the print medium by dropping or ejecting the ink from the printhead, or by any other printing method well known by those skilled in the art. Ink used in wet ink type printing includes a relatively large amount of water. As the wet ink contacts the print medium, the water in the ink saturates the fibers of the print medium, causing the fibers to expand, which in turn causes the print medium to buckle. Buckling of the print medium tends to cause the print medium either to bend downwardly away from the printhead, or to bend upwardly toward the printhead. In either case, a constant pen-to-print medium spacing is not achieved, leading to poor print quality. Additionally, upwardly buckling print medium may contact a pen nozzle, leading to ink smearing on the print medium.
Typically, to achieve good print quality, pen-to-print medium spacing of less than 1.5 millimeter (ram), and preferably less than 1.0 mm, is required. However, bending amplitudes of print medium in certain pen/ink combinations can be greater than 3 mm. To reduce this problem of paper buckling, which varies the pen-to-print medium spacing, various shaped platens were designed.
The Hewlett-Packard Deskjet (a trademark of Hewlett-Packard) printer includes a platen with a flat print medium contacting surface and a feed mechanism, usually a drive roller, positioned adjacent the platen. The flat expanse of the platen is positioned below the printhead such that the platen supports the print medium throughout a print zone defined between the printhead and the platen. The feed mechanism is positioned such that print medium is fed at a downward angle onto the platen such that the print medium is concavely curved relative to the printhead in an initial region of a print zone. This small region of concave curvature generally does not extend under the pen nozzles of the printhead. Thus, when the print medium does not buckle, the print medium merely lies flat against the contacting surface of the platen throughout the print zone. However, when the print medium buckles during printing the flat platen prevents the print material from buckling downwardly away from the printhead and so the print medium is forced to buckle upwardly toward the printhead. Thus, the Deskjet device does not adequately ensure proper pen-to-print medium spacing. In addition, the device increases the risk of ink smearing due to possible pen-to-print medium contact when the print material buckles upwardly.
The Hewlett-Packard Paintjet XL (a trademark of Hewlett-Packard) printer includes the elements of the Deskjet printer, but also includes a second drive roller positioned adjacent an exit area of the print zone. Thus, print media are fed downwardly onto the platen from the feed mechanism, or first drive roller, extend throughout the print zone, and then travel over the second drive roller, such that the print medium are positioned between the drive roller and an adjacent star wheel. The second drive roller is positioned generally above the platen such that the first drive roller and the second drive roller may effect a generally concave curve in the print medium relative to the printhead, throughout the print zone. Since the print medium is gripped between a paper guide and the first drive roller on one side of the printhead, and the print medium is gripped between the second drive roller and a star wheel on another side of the printhead, the sheet of print medium is held in a controlled curve throughout the print zone. This controlled curve ensures proper pen-to-print medium spacing during printing, thereby ensuring good quality printing. However, inclusion: of the second drive roller in the Paintjet XL printer increases the cost and complexity of the printer. Also, the possibility of ink smearing is increased due to the star wheel contacting the freshly printed print medium, such that the star wheel presses the print medium against the second drive roller. Additionally, intake problems can arise when sheets of print media are improperly fed between the second drive roller and the star wheel.
The Hewlett-Packard Designjet (a trademark of Hewlett-Packard) printer includes a driver roller positioned beneath a printhead, the drive roller acting as a rotating platen. Specifically, sheets of print medium are fed through a print zone defined between the drive roller and the printhead. The sheets are held in contact with the curved outer surface of the drive roller on one side of the printhead by a paper guide positioned adjacent the roller, and on the other side of the printhead by a star wheel positioned adjacent the drive roller. In this arrangement, print medium are held in a generally convexly shaped curve relative to the printhead throughout the print zone. However, due to the curved surface of the drive roller, the print zone in such Designjet printers must be relatively narrow to achieve an acceptable pen-to-print medium spacing. For example, a drive roller having a radius of 31.75 mm (1.25-inches) ensures adequate print medium bending control, but would require a small print zone, and therefore a short printing array of ink nozzles in the printhead. A 12.70 mm (1/2-inch) printing array, for example, in combination with a 31.75 mm (1.25-inch) radius roller would result in a 0.63 mm (0.02-inches) change in pen-to-print medium spacing due to the drive roller curvature alone. Additionally, the possibility of ink smearing is increased due to the star wheel contacting the freshly printed medium, and forcing the medium against the surface of the drive roller.